A group call is a conference call in which all participants may talk in turn and hear one another. In group calls, the whole group is called with a single dialled subscriber number, called a group number herein. An individual radio unit (e.g. mobile radio) may belong to a plurality of groups programmed into the radio unit. The system keeps a file on the base stations associated with the group number of each group. A group call may cover one, several or all base stations within the area of a mobile exchange or a plurality of mobile exchanges. In setting up a group call, a traffic channel is allocated from all base stations associated with the group, and each of these base stations transmits a group call paging message containing the group number and an indication of the allocated traffic channel. If the mobile radio identifies the group number contained in the group call paging message, it switches to the traffic channel indicated by the group call paging message. Therefore, it is always possible for a mobile radio station to join the group call if it is roaming in the predetermined operation area of the group.
In a mobile radio system operating on a cellular principle, adjacent base stations often have considerably overlapping coverage areas (radio cells). Mobile radios which are situated in a part of a coverage area shared by two base stations may use either one of the base stations. As regards a group call, the situation is problematic if one base station is located within the predetermined operation area of a given group and the other outside it. If a group call is set up in the overlapping area of such coverage areas, mobile radios of the same group may utilize different base stations, whereby only some of them join the group call, although in principle all are in the operation area reserved for the group.
Attempts have been made to solve this problem by setting up the group call, just in case, through more base stations than actually required for covering the operation area of the group. Traffic channels for the group call are also allocated from such `extra` base stations. This solution, however, leads to inefficient use of traffic channels in the system and may even be impossible in some systems for lack of traffic channels.
The object of the invention is a method for setting up regional group calls without the abovementioned disadvantages.